For many applications, the light from an LED must be input into an optical system which is arranged downstream. This optical system usually has a specific numerical aperture (NA), i.e. a maximum angle of incidence relative to the axis of the system. Light which is incident at relatively large angles with respect to the axis cannot be used in this optical system and in the most unfavorable case gives rise to scattered light and artifacts.
In addition, applications such as, for example, LCD back lighting with LEDs or LEDs as light sources in projectors frequently require polarized light. However, the emission from LEDs is based on spontaneous emission and is therefore non-polarized.
In general terms, it is desirable to implement LEDs with optical properties which exceed normal optical properties without the need to accept significantly decreased efficiency of the LEDs.